GENERALIZA TfON 245 



on the Inverse Method of Probabilities (vol. i. pp. 276- 

 312), that whenever in the future we meet an object pos- 

 sessing either one of the properties of gravity and inertia, 

 it will be found on examination to possess the other of 

 these properties. This is a clear instance of the employ- 

 ment of generalization. 



In analogy, on the other hand, we reason from likeness 

 in many points to likeness in other points. The qualities 

 or points of resemblance are now numerous, not the 

 objects. At the poles of Mars are two white spots 

 which resemble in many respects the white regions of 

 ice and snow at the poles of the earth. There probably 

 exist no other similar objects with which to compare 

 these, yet the exactness of the resemblance enables us 

 to infer, with high probability, that the spots on Mars 

 would be found to consist of ice and snow, if we could 

 examine them. 



In short, many points of resemblance imply many more. 

 From the appearance and behaviour of those white spots 

 we infer that they have all the chemical and physical 

 properties of frozen water. The inference is of course only 

 probable, and based upon the improbability that aggregates 

 of many qualities should be formed in a like manner in 

 two or more cases, without being due to some single 

 uniform condition or cause. In reasoning by analogy, 



then, we observe that two objects ABODE and 



A' B' C' D' E' have many like qualities, as indicated 



by the identity of the letters, and we infer that, since the 

 first has another quality, X, we shall also discover this 

 quality in the second case by sufficiently close examina- 

 tion. As Laplace says, ' Analogy is founded on the 

 probability that similar things have causes of the same 

 kind, and produce the same effects. The more perfect this 

 similarity, the greater is this probability' 13 . The nature 

 b ' Essai Philosophique sur les Probabilities/ p. 86. 



